Here, I've attempted to gather together some useful
photopopper
circuits. Note that most photopopper
circuits are based on just a pair of solar
engine circuits wired together ("back to back). This
means you can get additional circuits by replacing solar
engines in the diagrams below with other solar
engine circuits. Generally in such cases, it's good to
incorporate a variable resistor in your circuit so that you
can "tune" the two engines for balanced behavior (else, due
to manufacturing variances in component values, your
photopopper
will tend to move in curves).